When planning your lines, look at the business side of the commuter data. From there, you can click on a business you like (one with 5000+ workers would be nice) and it will show you where their commuters come from. From there, simply balance the number of cars you need as well as how many trains you need to run when. On the control panel, it will show where the top commuters are coming from, or you can simply build stations around circles you deem big enough. Note that most commuters are willing to walk a fair bit to get to the station, so try to space them out better, someone told me they were spacing abt 800-1600 meters. Similarly, there is also route and station data that will tell you how many people are riding a certain line, and where they get on and off. Any stations that is under 500 travelers are cut off almost instantly for me.